The first oxalate-based 2D coordination polymer containing the biomimetic ligand 5-methyl-1,2,4-triazolo[1,5-a]pyrimidin-7-one

Article abstract of DOI:10.1016/j.inoche.2012.01.031

Reaction of 5-methyl-1,2,4-triazolo[1,5-a]pyrimidin-7(4H)-one (HmtpO) with manganese(II) nitrate in presence of oxalic acid yielded the first complex containing this hypoxanthine analog and oxalate, namely [K₂Mn ₂(μ-mtpO)₄(μ₆-ox)(μ-H ₂O)₆]_n, where mtpO is the anionic form of HmtpO derivative. The X-ray structure of this bimetallic polymer is built up of metal-oxalate cationic chains which are further linked by mtpO ligand in an unusual fashion to give a neutral 2D framework.

Full text of DOI:10.1016/j.inoche.2012.01.031

Inorganic Chemistry Communications 19 (2012) 36–38
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
The first oxalate-based 2D coordination polymer containing the biomimetic
ligand 5-methyl-1,2,4-triazolo[1,5-a]pyrimidin-7-one
Ana B. Caballero , Óscar Castillo , Luis Lezama , Antonio Rodríguez-Diéguez a,1, Juan M. Salas a,⁎
a,1
b
b
a
Departamento de Química Inorgánica, Facultad de Ciencias, Universidad de Granada, E-18071, Granada, Spain
Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco (UPV/EHU), Apdo. 644, E-48080 Bilbao, Spain
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of 5-methyl-1,2,4-triazolo[1,5-a]pyrimidin-7(4H)-one (HmtpO) with manganese(II) nitrate in
presence of oxalic acid yielded the first complex containing this hypoxanthine analog and oxalate, namely
[K Mn (μ-mtpO) (μ -ox)(μ-H O) ] , where mtpO is the anionic form of HmtpO derivative. The X-ray structure
2 2 4 6 2 6 n
of this bimetallic polymer is built up of metal-oxalate cationic chains which are further linked by mtpO ligand in
an unusual fashion to give a neutral 2D framework.
Received 21 September 2011
Accepted 23 January 2012
Available online 31 January 2012
Keywords:
©
2012 Elsevier B.V. All rights reserved.
Manganese(II) complex
Triazolopyrimidine
Oxalate
X-ray structure
Thermal analysis
Magnetic susceptibility
Over the past decades, notable research effort has been devoted in
the rational design and synthesis of biomimetic systems based on the
interaction of biologically relevant molecules with a wide range of
organic and inorganic species [1]. The interest in these systems not
only stems from the desire to better understand the complex interac-
tions often present in a great diversity of molecular biorecognition
processes [2] but also to afford a powerful tool for the improvement of
pharmaceutical agents [3] and the development of artificial receptors
used as specific nucleotide sensors or even for the determination of
low concentrations of biological and therapeutic agents [4]. In this
background, nitrogen heterocycles known as 1,2,4-triazolo[1,5-a]
pyrimidines may be used as purines analogs to pursue a deeper under-
standing of interactions between purine nucleobases and metal ions,
and to obtain new biomimetic systems with interesting properties.
The slightly different atomic arrangement in 1,2,4-triazolo[1,5-a]
pyrimidines may introduce new structural features and different phys-
ical and biological properties compared to purine-based systems [5,6].
Previous studies carried out by our research group, among others,
have revealed that 1,2,4-triazolo[1,5-a]pyrimidines are versatile
ligands as they have several nitrogen atoms to bind metal ions [5,7].
Moreover, their versatility can be increased by the ring-substitution
with functional groups containing donor atoms like nitrogen, sulfur or
oxygen, leading to a wider range of coordination modes and structural
topologies in their metal complexes [5–8]. Therefore, an increasing
interest has recently aroused around this type of ligands due to their
great capacity to act as suitable building blocks for the synthesis of
novel metal-organic multidimensional systems, [9,10] showing in
some cases interesting magnetic, luminescent and biological properties
[5].
On the basis of the aforementioned considerations, we have chosen
the substituted derivative 5-methyl-1,2,4-triazolo[1,5-a]pyrimidin-
7(4H)-one (HmtpO) in combination with a bridging and versatile
ligand such as the oxalate anion (ox2 ) to obtain novel multidimen-
sional metal complexes which may show unusual physical and biologi-
−
cal properties. As a result, a novel 2D bimetallic polymer, [K
mtpO) (μ -ox)(μ-H O) , where mtpO is the anionic form of HmtpO,
has been obtained under conventional reaction in aqueous media,
2 2
Mn (μ-
4
6
2
6 n
]
2
and has been characterized by IR spectroscopy, elemental analysis,
single-crystal X-ray diffraction and thermal analysis. Magnetic suscepti-
bility measurements have been also carried out. To the best of our
knowledge, this is the first oxalate compound which contains such
nucleobase analog, and it is part of our preliminary results on a recently
2
3 2 2
Synthesis: Over an aqueous solution of Mn(NO ) ·4H O (0.3 mmol, 0.075 g,
10 mL), 20 mL of an aqueous solution containing HmtpO (0.6 mmol, 0.0901 g) and
KNO (0.3 mmol, 0.030 g) was added. The mixture was heated at 50 °C for 10 min.
3
Then, the resulting clear solution was acidified with diluted HCl at pH 0, and over this
solution was added dropwise an aqueous solution of oxalic acid dihydrated (0.3 mmol,
0.038 g, 10 mL) keeping stirring for 30 min. Finally, pH value was raised to 7 by adding
diluted KOH very slowly, during one hour approx. (to avoid quick formation and pre-
cipitation of manganese oxalate, manganese hydroxide or the binary complex with
mtpO). The resulting solution was kept stirring for one hour and then was left evapo-
rating at room temperature. After 4–5 days, crystals of manganese(II) oxalate were
formed and filtered off and, a few days later, colorless crystals of the title compound
⁎
Corresponding author. Tel.: +34 958248525.
E-mail address: jsalas@ugr.es (J.M. Salas).
Tel.: +34 958248525.
were isolated. (Yield: 20%). Elemental analysis data: % calc. for C26
(% found): C, 31.84 (32.02); H, 3.29 (4.26); N, 22.85 (22.04).
32 2 2 16 14
H K Mn N O
1
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.01.031

A.B. Caballero et al. / Inorganic Chemistry Communications 19 (2012) 36–38
37
Fig. 1. View of the coordination environment of the metal atoms, along with the atom
numbering scheme. It is also clearly shown the bridging fashion of oxalate ligand (50%
probability ellipsoids). Hydrogen atoms are omitted for clarity.
Fig. 2. Two-dimensional framework along a axis direction.
mtpO ligands, two oxygen atoms of an oxalate ligand and two water
molecules. Cis angle values are ranging from 80.5(2)° to 109.7(2)°,
while for manganese center, they vary from 74.5(2)° to 96.4(2)°.
Fig. 1 also shows the simultaneous coordination of the oxalate anion
started approach to design new biomimetic systems which may show
either physical or biological properties for their use as chemotherapeu-
tic agents.
to two Mn2 ions and four K ions while two water molecules are
connecting two neighboring potassium ions; it should be noted that
the chelating effects of both oxalate and water ligands are the main
responsible for the distortion in the coordination geometry of both
metal atoms. In this way, both oxalate and water bridges sequentially
connect metal centers to form a linear 1D polymeric ribbon. These
metal-oxalate cationic ribbons or double chains are further linked by
mtpO ligands acting as spacers (through their N3 and O7 atoms) to
result in a neutral 2D framework as shown in Fig. 2 (see also Fig. S1).
This coordination mode for the triazolopyrimidine derivative is rather
unusual since it has only been observed previously in the coordination
+
+
The infrared spectrum of the title compound shows two intense
−
1
bands at 1637 and 1528 cm , which are displaced to lower frequen-
cies than those for the neutral form of HmtpO, due to the ligand depro-
tonation. These bands may be assigned to a combination of stretching
vibrations of C_O bond and pyrimidinic and triazolic rings C_C and
C_N bonds: ν(C_O)+ν(pym)+ν(trz) [11]. The stretching vibration
of the oxalate C_O bonds is not observed as may be overlapped with
those of the triazolopyrimidine ligand. The IR spectrum also shows a
−
1
broad band around 3400 cm
tions of the water molecules.
due to the ν(O\H) stretching vibra-
3
According to single crystal X-ray diffraction studies, the structure
2 2 4
of [K Mn (μ-mtpO) (μ
6
-ox)(μ-H
2
O)
6
]
n
exhibits a 2D heterometallic
2 2 2 4 2 n
polymer {[Cu(μ-HmtpO) (H O) ](ClO ) ·2HmtpO} , which has been
+
2+
coordination framework in which K and Mn ions and three differ-
ent bridging ligands are coexisting, namely mtpO, oxalate and water
molecules. The asymmetric unit consists of two manganese (II) ions,
two potassium ions, one oxalate ligand, four mtpO ligands and six
water molecules. Both potassium and manganese ions display
distorted octahedral coordination geometry (Fig. 1). Each potassium
ion is coordinated to six oxygen atoms, two belonging to two different
oxalate anions, another two pertaining to two mtpO ligands and two
water molecules. The coordination environment of Mn(II) atoms is
recently reported by us [8]. These ligands are not paralelly arranged
but their molecular planes form a dihedral angle of ca. 65°, so there
are any π-stacking forces between aromatic systems. It should be
highlighted the rare coordination behavior of the oxalate ligand,
which is also quite unusual in metal-organic systems [12]. Selected
bond distances and intermetallic distances are listed in Table S1.
The resulting sheets are packed, just by weak van der Waals forces,
in an ABAB sequence. However, strong intra-sheet hydrogen bonds
establish between oxygen atoms of water and mtpO ligands and nitro-
gen atoms N1 and N4.
2 4
of MnN O type and formed by two nitrogen atoms (N3) of two
Regarding thermal behavior of the polymer [K
2 2 4 6
Mn (μ-mtpO) (μ -
ox)(μ-H O) , its dehydration takes place in the 90–170 °C tempera-
2
6 n
]
3
Crystal data: crystal diffraction intensities were collected, at room temperature, in
a Xcalibur diffractometer equipped with an area detector and graphite monochro-
mated MoKα (λ=0.71073 Å). The data reduction was done with the CrysAlis RED
software of the diffractometer. The structure was solved by direct methods using
SIR92 program and refined by full-matrix least-squares on F2 including all reflections
ture range in a well-defined weight loss effect in TG diagram, which
corresponds to the loss of all water molecules; the associated enthalpy
is −60.5 kJ per water mol (average value). The anhydrous compound
is stable until 300 °C; at this temperature decarboxylation begins
followed by the pyrolysis of the organic moiety, which finishes at
(
SHELXL97). All calculations were performed using the WINGX crystallographic soft-
ware package. During the data reduction process it became clear that the crystal spec-
imen was a non-merohedral twin, in which both parts of the twin are related by means
of a binary rotation axis along the [100] direction (twin law: 1 0 0/0.5–1 0/0–1 1).
Structure refinements were carried out through SHELX97 using HKLF5 and BASF en-
tries, which revealed a fractional value of 0.35 for the minor domain.
All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of the het-
erocycles were placed in ideal positions with Uiso 20% larger than Ueq for the atoms to
which they are attached. H atoms belonging to the methyl group and to the water mol-
ecules were located in the difference Fourier maps and kept fixed at these positions
7
70 °C. The inorganic residue is a mixture of MnO
The magnetic characterization of the title compound was carried
out through measurements of the variation of the molar magnetic sus-
2 2
and KO .
4
ceptibility, χ
m
, with temperature (Fig. S2). The χ
m
T value continuously
T at
K (μeff=8.24 BM), which is in
decreases on cooling, more quickly below 100 K. The value of χ
room temperature is 8.48 cm mol
m
3
−1
good agreement with that expected for two isolated Mn(II) ions of
with Uiso 50% larger. Crystal dimensions 0.42×0.18×0.15 mm, formula C26
14, fw 1422.84, triclinic, space group P–1, a=8.0349(4) Å , b=15.1016(8) Å,
c=16.6821(8) Å, α=106.118(4)°, β=90.056(4)°, γ=97.687(4)°, V=1925.62(17)
32 2
H K Mn2-
16
N O
4
Magnetization and variable temperature (1.9–300 K) magnetic susceptibility mea-
3
−3
−1
Å , Z=2, T=293(2), ρcalcd=1.691 g cm , μ=0.958 mm , F(000)=1000, 6612
surements on polycrystalline sample were carried out on a Quantum Design SQUID
MPMS-7 device operating at 0.1 T. The experimental susceptibilities were corrected
for the diamagnetism of the constituent atoms by using Pascal's tables.
2
reflections collected, 6612 unique (Rint=0.0000). Goodness of fit on F =1.079,
2
R=0.0706 (I>2σ(I)), wR =0.1903 (all data).

38
A.B. Caballero et al. / Inorganic Chemistry Communications 19 (2012) 36–38
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the magnetic effective moment at low temperatures are in agreement
with weak antiferromagnetic interactions in the compound. Taking
into account the crystal structure the experimental data were fitted to
a model of S=5/2 dimers, from which a satisfactory fit was obtained
(
−
1
with a coupling constant (J) value of −1.76 cm
between manganese ions through oxalate bridging ligand. The obtained
J value lies in the lower range found for others trans-MnO chromo-
phores bridged by bisbidentated oxalato ligands (−2.16bJb
for the interaction
(
4 2
N
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